Communication
sponding zero-valent ethylene-coordinated complexes of NHC-
P, dcype, and bipyridine were performed, and the HOMO
energy level of the NHC-P complex was found to be the high-
est among the three (for details, see the Supporting Informa-
tion).
2
Table 1. Acrylate salt synthesis from ethylene and CO .
Next, the cleavage of nickelalactone 2a was examined
step B, Scheme 1). On treatment of 2a/2a’ with two equiva-
Entry
Cat.
Additive
TON
6.9
34
58
73
94
12
31
(
[
[
a]
1
2
3
4
5
6
7
8
9
1
1a
1a
1a
1a
1a
1a
1a
1a
none
none
none
lents of tBuOK in [D ]THF, acrylate salt-coordinated complex 3a
8
b]
was generated as a single major species within a few minutes
[15]
at room temperature (Scheme 3). Such reactivity was similar
[c]
Zn
[5a]
to the previously reported nickel systems.
AcONa
PhCOONa
CF COONa
3
HCOONa
AcONa
AcONa
AcONa
AcONa
AcONa
AcONa
6.1
2.3
3.2
1b
1c
1d
0
1
1
140
1
1
1
2
3
4
[Ni(dcype)(cod)]
[Ni(bpy)(cod)]
1d
85
0
450
[d]
[e]
Conditions: Ni catalyst (0.01 mmol), THF 2.0 mL, phenoxide (3.0 mmol),
(1.5 MPa), CO (1.5 MPa), additive (3.0 mmol). TON of sodium acrylate
was determined by H NMR in D
panesulfonate as an internal standard. [a] 1008C. [b] 1208C. [c] 1.0 mmol.
d] Ni catalyst (0.005 mmol), phenoxide (6.0 mmol, 1200 equiv to Ni),
Scheme 3. Cleavage of nickelalactone (2a) by potassium tert-butoxide.
C
2
H
4
2
1
2
O using sodium 3-(trimethylsilyl)-1-pro-
Finally, replacement of the coordinated acrylate salt by ethyl-
ene was investigated (step C, Scheme 1). When the acrylate
salt-coordinated complex 3a was generated in situ from nick-
[
NaOAc (6.0 mmol, 1200 equiv to Ni). [e] Average of three experiments.
elalactone 2a/2a’ and tBuOK in [D ]THF and was exposed to
8
1
atm of ethylene, the ligand exchange reaction was found to
proceed rapidly within 15 minutes at room temperature to
duced to a THF solution of a nickel complex (0.01 mmol) and
[
16]
give ethylene-coordinated complex 4a and potassium acrylate
sodium 2,4,6-trimethylphenoxide (3.0 mmol) as a base, and
the mixture was stirred for 8 hours. When the reaction was
performed with complex 1a at 100 or 1208C, acrylate salt was
obtained with a TON of 6.9 and 34, respectively (Table 1,
entry 1, 2). On the other hand, when the reaction was per-
formed at 1458C, TON was increased up to 58 (entry 3). Such
dependency on temperature was different from the previously
reported BenzP*/nickel system, for which 100 or 1208C gave
the best results and a decrease of TON was observed at
(
about 80 and 70% from 2a/2a’, respectively, Scheme 4). This
[
5c,d]
1
458C.
These results indicate that complex 1a is intrinsically
less active as a catalyst compared to the previously reported
bisphosphine/nickel system, but it is thermally robust so that a
Scheme 4. Exchange of acrylate salt with ethylene.
[
8f]
higher temperature is applicable.
is much faster than the previously reported example for
In order to enhance the TON, effect of additive was investi-
gated. In contrast to the previously reported bisphosphine/
[
Ni(dtbpe){Na(acrylate)}] (dtbpe=1,2-bis(di-tert-butylphosphi-
no)ethane) (96% conversion, with 8 bar of ethylene, 608C,
nickel systems, TON only slightly increased to 73 by the addi-
[
5a]
[5b,c,d]
2
0 h),
and is another advantageous aspect of the NHC-P
tion of metallic zinc
(Table 1, entry 4). With the assumption
ligand. With the assumption that the reaction would proceed
by an associative mechanism through an intermediate coordi-
that cleavage of nickelalactone is the rate-determining step,
other additives were investigated in addition to 2,4,6-trimethyl-
phenoxide. Previously, it was reported that alkaline metal cat-
ions could work as a Lewis acid to facilitate the cleavage of
[4c,5a]
nated by both ethylene and acrylate salt,
the acceleration
effect of the NHC-P ligand should be due to either rapid coor-
dination of ethylene to acrylate salt-coordinated complex or
rapid dissociation of K(acrylate) from the tetra-coordinated in-
termediate. Although speculative, the strong trans effect and
the steric hindrance of the NHC ligand might be accelerating
the latter dissociation step.
[
3d,4c,e,g,5a,b]
nickelalactone.
Then we investigated several alkaline
metal salts as an additive, and AcONa was found to be an ef-
fective additive (entry 5). In the presence of 3.0 mmol of
AcONa, TON increased to 94, whereas other kinds of sodium
carboxylates gave lower TON (entries 6–8). These results indi-
cate that the positive effect of AcONa was not only originated
from Na cation, but also from acetate anion. Moreover, use of
Based on the information obtained from the stoichiometric
reactions described above, the catalytic activities of NHC-P
nickel complexes were investigated (Table 1). As standard con-
F
NaBAr did not give a better result (see the Supporting Infor-
ditions, 3.0 MPa of a 1:1 mixture of ethylene/CO was intro-
mation). One possible effect of AcONa is the tuning of the pH
2
Chem. Eur. J. 2019, 25, 13504 – 13508
13506
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